1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method for transferring visible images formed on plural image carriers to the endlessly-moving front surface of an endless movement body or a recording member held on the front surface thereof so as to be superimposed one on another.
2. Description of the Related Art
A known image forming apparatus of this type is one described in Patent Document 1. This image forming apparatus has plural photosensitive bodies serving as image carriers and a belt member serving as an endless movement body that performs an endless movement to successively pass through positions facing the photosensitive bodies. In the image forming apparatus, toner images of different colors are formed on the front surfaces of the respective photosensitive bodies and then transferred to a recording paper held on the front surface of the belt member so as to be superimposed one on another through an electrophotographic process. With the superimposition of the toner images, a multicolor toner image is formed on the front surface of the recording paper.
In the image forming apparatus that forms the multicolor toner image in this manner, displacements in the superimposition of the dots of respective colors may occur due to the off-centering of a photosensitive-body gear fixed to the rotary shaft of a photosensitive body. Specifically, when the maximum-diameter part of the photosensitive-body gear, at which a length from the rotary shaft to a gear tooth tip is the longest with respect to the off-centering of the photosensitive-body gear, is meshed with a driving gear, the linear speed of the photosensitive body per rotation becomes the slowest. Conversely, when the minimum-diameter part of the photosensitive-body gear, at which the distance from the rotary shaft to the gear tooth tip is the shortest with respect to the off-centering of the photosensitive-body gear, is meshed with the driving gear, the linear speed of the photosensitive body per rotation becomes the fastest. Since the maximum- and minimum-diameter parts of the photosensitive-body gear are symmetrical about a point by 180° relative to the rotary shaft, the linear speed of the photosensitive body has a fluctuation characteristic in which a sine curve for one cycle is displayed per cycle of the gear. Among mountain parts and valley parts created when a center line is drawn between the maximum and minimum values of the sine curve, the valley parts of the sine curve indicate that the linear speed of the photosensitive body becomes slower than its original speed (the value of the center line). Thus, dots on the photosensitive body are transferred to a recording paper moving faster than the photosensitive body in a state of being more expanded than usual in a belt movement direction. On the other hand, the mountain parts of the sine curve indicate that the linear speed of the photosensitive body becomes faster than its original speed. Thus, the dots on the photosensitive body are transferred to the recording paper moving slower than the photosensitive body in a state of being more contracted than usual in the belt movement direction. The dots of the respective colors are thus separately expanded and contracted to cause the displacements in the superimposition of the dots of the respective colors.
The image forming apparatus described in Patent Document 1 prevents the displacements in the superimposition of the dots of the respective colors in the following manner. In other words, a pattern image for detecting a speed fluctuation waveform is first formed on the belt member at a predetermined timing. This pattern image is composed of patch-like toner images of the respective colors arranged in a belt-surface movement direction in a predetermined order. If no speed fluctuation occurs in the photosensitive bodies for the respective colors, the toner images are arranged at even intervals. On the other hand, if any speed fluctuation occurs in the photosensitive bodies for the respective colors, the toner images are not arranged on the belt member at even intervals and differences between the intervals reflect the speed fluctuation of the photosensitive bodies for the respective colors. The differences between the intervals are detected based on an output from a photosensor that detects the toner images for the respective colors, whereby the sine-curve-like speed fluctuation waveform is detected for each of the photosensitive bodies for the respective colors. Then, the driving speed variation pattern of each of the photosensitive bodies capable of canceling the sine-curve-like speed fluctuation waveform is determined and stored in a data storage unit. Subsequently, when a print job is performed, reference timing in a one-rotation cycle is recognized for each of the photosensitive bodies for the respective colors based on an output from an encoder fixed to each of the rotary shafts of the photosensitive bodies for the respective colors. Then, the driving speed of each of plural driving motors that separately drive the photosensitive bodies for the respective colors is finely adjusted based on the reference timing and the driving speed fluctuation pattern stored in advance. With this fine adjustment of the driving speed, the speed fluctuations of the photosensitive bodies for the respective colors are reduced, so that the displacements in the superimposition of the dots of the respective colors can be prevented.
Note that in addition to the displacements in the superimposition of the dots due to the speed fluctuations of the photosensitive bodies, the image forming apparatus described in Patent Document 1 detects displacements in the superimposition of the dots due to the speed fluctuation of a driving roller that drives the belt member, or the like. For this reason, the above-described pattern image is formed. However, when only the speed fluctuations of the photosensitive bodies are detected to control the driving motor, the formation of the pattern image can be omitted. This is because the speed fluctuation waveforms of the photosensitive bodies can be detected based on the outputs from the encoders fixed to the rotary shafts of the photosensitive bodies.
The image forming apparatus described in Patent Document 1 refers to the displacements in the superimposition of the dots on the recording paper conveyed in a state of being held on the front surface of the belt member, but a similar displacement in the superimposition of the dots may occur in an image forming apparatus having the following configuration. In other words, the image forming apparatus is configured to transfer the toner images formed on the photosensitive bodies for the respective colors to an intermediate transfer belt serving as the belt member so as to be superimposed one on another and then transferring them to the recording paper in a collective manner.
On the other hand, Patent Document 2 describes an image forming apparatus that reduces the displacements in the superimposition of the dots of the respective colors by matching the phases of the speed fluctuation waveforms of the photosensitive bodies for the respective colors to each other. Specifically, in this image forming apparatus, the arrangement pitches of the photosensitive bodies are set to be an integral multiple of the circumferential length of the photosensitive bodies. Under this setting, the photosensitive-body gears are rotated by an integral number while the belt member is moved from one position facing any one of the photosensitive bodies to another position facing the adjacent photosensitive body. Therefore, when the photosensitive bodies are rotated in a state in which rotational phases at the maximum- and minimum-diameter parts of the photosensitive-body gears are matched to each other, the two adjacent photosensitive bodies transfer the following dots to the recording paper so as to be superimposed one on another. In other words, the dots entering a transfer position when the photosensitive bodies are driven at the maximum linear speed per rotation and those entering the transfer position when the photosensitive bodies are driven at the minimum linear speed per rotation are transferred to the recording paper. In such the superimposition of the dots, the dots transferred in a state of being more expanded than usual and those transferred in a state of being more contracted than usual due to the speed fluctuations of the photosensitive bodies are superimposed one on another, thereby making it possible to prevent the displacements in the superimposition of the dots. To this end, the image forming apparatus described in Patent Document 2 has plural photosensors that detect marks provided at the maximum- and minimum-diameter parts of the photosensitive bodies at predetermined positions. Furthermore, the image forming apparatus controls the driving of the driving motors for the respective colors so that timings for detecting the marks with the photosensors are synchronized with each other.
Patent Document 1: JP-B2-3186610
Patent Document 2: JP-A-2003-194181
Meanwhile, since reduction of manufacturing costs for apparatuses has been in demand recently, it is desired in some cases that the driving motor for driving the photosensitive body be also used as a driving source for members other than the corresponding photosensitive body. For example, it is desired in some cases that, regardless of a monochrome mode and a color mode, the driving motor for black to be driven among plural of the driving motors corresponding to the photosensitive bodies for the respective colors be used as a driving source for the belt member. In this case, when the driving speed of the driving motor for black is finely adjusted based on the predetermined driving speed variation pattern to cancel the speed fluctuation due to the off-centering of the photosensitive body for black, the fluctuation of the speed of the belt member occurs. Accordingly, processing for finely adjusting the driving speed of the driving motor based on the driving speed variation pattern as described in Patent Document 1 is not suitable. Conversely, when performing the processing for matching the rotational phases of the photosensitive-body gears for the respective colors to each other as described in Patent Document 2, the image forming apparatus can prevent the displacements in the superimposition of the dots of the respective colors without causing the speed fluctuation of the belt member described above.
However, while high quality has been in demand in recent technologies, the displacements in the superimposition of the dots of the respective colors exceeding a tolerance level may remain only if the rotational phases of the photosensitive gears for the respective colors are matched to each other. Specifically, the displacements in the superimposition of the dots of the respective colors cannot be eliminated only with the superimposition of the rotational phases of the photosensitive-body gears for the respective colors. It is assumed that speed fluctuation occurs in the photosensitive bodies for the respective colors as shown in FIG. 1 when the toner images of yellow (Y), magenta (M), cyan (C), and black (K) are formed on the separate photosensitive bodies and then transferred to the recording paper. In an example shown in FIG. 1, the speed fluctuation of the photosensitive body for yellow among the four photosensitive bodies corresponding to the respective colors is the greatest. Furthermore, the speed fluctuation of the photosensitive body for cyan is the smallest. As shown in FIG. 1, when the rotations of the respective photosensitive bodies are controlled so that the speed variation waveforms of the respective photosensitive bodies are matched to each other, the dots transferred at the maximum linear speed and those transferred at the minimum linear speed of the photosensitive bodies for the respective colors are superimposed one on another. As a result, the amounts of the displacements in the superimposition of the dots can be reduced. However, as shown in FIG. 2, the displacement in the superimposition of the dots corresponding to the difference between the amplitude of the speed variation waveform of the photosensitive body for yellow having the greatest speed fluctuation and that of the speed fluctuation waveform of the photosensitive body for cyan having the smallest speed fluctuation among the four photosensitive bodies remains. When the difference between the amplitudes is relatively large, the amount of the displacement in the superimposition of the dots exceeds a tolerance level.